Synonymy of Thysanus and Signiphora is discussed as well as classification of species of Signiphora. A new species, Signiphora borinquensis Quezada, DeBach, and Rosen, is described and its morphology, biology, behavior, and ecology treated in detail. Signiphora borinquensis is a thelyotokous, primary endo-ectopara-site of certain diaspine scale insects. Size of parasites as well as size of hosts influence both total number and size of resulting progeny. Host-feeding is prerequisite to oviposition, and both occur within one hour after adult emergence. Over half of the total egg production occurs during the first three days. Honey (carbohydrate) and hosts (protein) are constantly necessary to insure optimum longevity and progeny production. High host specificity is indicated. Males are extremely rare and unnecessary for reproduction, but their incidence can be greatly increased by subjecting female pupae to 90°F for 48 hours. The resulting adult females tend to produce males. Ovisorption occurs under stress, but does not operate to maintain maximal progeny production. Effects of parasite density on longevity, progeny production, and on host survival are discussed.